That varies by case. There is no specified height for the stand offs.
Just take some standoffs you want to use, attach them to the board. Clip on the I/O shield for your motherboard. Measure from the bottom of the shaft of the stand off up to the i/O shield. (bascially everything but the exposed threads. That will fix the 'bottom' of the I/O shield and you can go from there.
Here is a good template for the I/O shield
http://www.tomshardware.com/forum/307917-30-case-shield-dimensions